题目内容：

Mary had finished her homework ____ the time I got home.
A.until B.by C.at D.when

优质解答

Mary had finished her homework （by,截止） the time I got home.
我到家的时候,玛丽已经完成作业了.
by the time+从句,表示截止到某时,在完成时态中作时间状语.